The present invention generally relates to fusing wire terminations and more particularly to fusing wire windings to terminals on devices such as electrical bobbins.
Electrical bobbins have been used since the 1920""s in a wide variety of applications. Typically, bobbins include a Bakelite or plastic cylindrical body usually defining a central, longitudinal bore. Fore and aft flanges give the body a spool shaped appearance. A pair of blade terminals are secured to the fore portion of the body on the fore flange or extensions of the fore flange and usually extend parallel to each other and either longitudinally forward of or transversely the body. These blades function as male terminals and removably insert into female receptacles as desired to make electrical contact therewith. In some bobbin designs the body end of each blade terminal typically forms a wing, arm, or stud extending laterally from the body. In others the terminals are molded into the flange itself. Still others include a U shaped plastic piece in which the legs of the U form the flanges and the central piece of the U runs parallel to the body axis and the terminals can be molded in the U central portion or extend from it if secured to some other part.
During bobbin manufacture, a winding machine winds an insulated wire a predetermined number of wire turns around the body mid-portion to form a coil. The coil is cut from the wire source so that the coil has two free distal ends. Each free wire end is conventionally welded or soldered to a respective blade terminal or stud extending from the terminal, thereby completing the electrical circuit from one blade terminal, through its stud, through the coil winding on the body, through the other stud and to other blade terminal.
Early bobbin making processes included soldering the distal winding ends to the studs. The soldering required pre-stripping the winding insulation and adding (melting) liquid solder to form the joint. However, in the 1940""s, there was developed a welded joint that did not require solder or pre-stripping the insulation. See FIG. 10. This technique used two upper electrodes (a main electrode (E) and auxiliary electrode (A)) in contact with each other. The work (W) was placed on a low resistant lower electrode (LE). Application of power was placed across the two upper electrodes to heat the main electrode (E). The wire insulation (I) held open the current path to the lower electrode (LE) open until the main electrode (E) burned off the wire insulation (I) at which time current flowed from the upper main electrode through the metal wire/stud (MW/S) to be joined and through the lower electrode (LE). The high current through the work heated the wire/stud further to melt the same and amalgamate the metals together.
Although this conventional method saves soldering and wire stripping time and materials and can provide a cleaner joint than soldering, it suffers from arcing and lack of careful control, repeatability and consistency from bobbin to bobbin. Accordingly, its yield percentage is not optimized.
Recently, a new technique has been developed to obtain better control over the fusing parameters and quality of the joint from bobbin to bobbin. This technique uses high resistance electrodes and passed the current only through the upper and lower electrode to heat the same individually. No current passes through the work. See FIG. 9. More specifically, the electrode E1, E2 were mounted on the electrode holders H1, H2 of a fusing machine similar to that disclosed in U.S. Pat. No. 4,371,772 and employing the method disclosed therein, except the current paths (I) were established as shown in FIG. 9. The electrodes E1, E2 had slot or cut-outs (S) to concentrate the current and heat at the zones near the electrode fusing surfaces that engage bobbin blades (13) and wire (WI) wound thereon. The vertical arrow indicated motion of electrode E1 toward and away from electrode E2.
Although this new technique improved some of the control parameters, it was not free from technical problems. For example, the electrodes tended to overheat and introduce defects in the fused joints. If thereafter operators continued to fuse joints, the electrodes would actually melt and sometimes cause damage to other parts of the machine. Also, the design of the electrodes could not accommodate the placement of bobbin wings that were set longitudinally aligned with or aft of the forward bobbin flange.
It is a primary object of the present invention to provide an apparatus and method for manufacturing devices, such as bobbins, in which the two wire terminations are formed simultaneously, with high quality and uniformity, and without the need to turn the work being so terminated. In addition, it is a primary object to effect such terminations with precise control of fusing electrode temperature at the fusing surface so that better control of the joint is achieved with an increase in manufacturing repeatability, reliability, quality and yields. Another primary object to prevent electrode overheating which may cause damage to the work and machine parts.
An exemplary embodiment according to the principles of the present invention includes using high resistance upper and lower fusing electrodes to provide great heat and high pressure to the wire/stud combination to burn-off the wire insulation, squeeze them under very high pressure, to form a mechanically fused or compression joint between each wire and stud. Fusing pressure and the electrode displacement during fusion is also be sensed and used in combination for various parameter control and quality checks.
As seen below, the electrodes are preferably shaped to concentrate the heat and pressure development precisely at the points of heat application to the contacted work. Advantageously, substantially no current passes through the small wire or from one electrode, through the wire to the other electrode. In addition, the upper electrode-contacting (pressure) surface is shaped to reduce strain between the compressed and uncompressed portion of the wire.
Also, the present invention provides much better control of the fusing process by first increasing pressure on the work and starting fusing power only when a predetermined pressure is reached. Power heats the electrodes to burn off insulation and soften but not melt the metal parts. An electrode zone immediately adjacent one of the fusing surfaces (preferably on the upper electrode) is monitored to control or pulse current to (i) control the fusing temperature for a period of time or until some other control parameter is reached and (ii) prevent electrode overheating. Time or the displacement of the electrode, which are still being pressed toward each other, can be used to determine cessation of the fusing power. This technique increases the yield, quality, and uniformity of the bobbin joints manufactured.
Both wire/stud sets on the bobbin can be fused simultaneously so that time is saved and turning fixtures and apparatus are not needed for a second fusing step on the same bobbin and the on-set and ceasing of fusing current can be precisely controlled relative to a predetermined electrode fusing surface temperature.